The present invention relates to a method of processing or upgrading electrical signals by means of a digital filtering system using Fourier transformation.
This kind of signal preparation relates to linear filtering or to signal improvement or to echo or feedback cancellation or to signal masking of an arbitrary combination of input signals. The preparation of signals in a digital filtering device is performed in discrete time intervals.
A discrete linear filtering operation can be expressed in a time interval by a discrete folding of a train of pulses u(k) with a pulse response h(k) of the filter, that is, by an equation ##EQU1## or in a frequency range by the multiplication of the input spectrum U(.OMEGA.) by a transfer function H(.OMEGA.), i.e. by an equation EQU Y(.OMEGA.)=U(.OMEGA.).multidot.H(.OMEGA.) (2)
whereby the frequency scale is compared with the scanning frequency f.sub.A (.OMEGA.=2.pi..multidot.f/f.sub.A).
Signal improvement is desired when an additive interference is superposed to the signal. This kind of interference can be caused by the acoustic background noise or due to electrical interference in a transmission channel. Provided that the stationary state of the interference is weak, then by means of a continuous parameter identification the stationary intervals, that is pauses in speech, and thus the parameters of the interference can be recognized. By means of the resulting parameters the filter coefficients of an optimum filter are computed (U.S. Pat. No. 4,025,721). This optimum filter minimizes the remaining errors. Another time-domain filtering method is known from German patent publication DE-AS 27 49 132, in which an adaptive, digital transversal filter is employed for the signal improvement, the filter coefficients being set according to a gradient algorithm, and the resulting evaluation signal should represent correct reproduction of the original signal. Still another known method for improvement of signals is based on the so-called spectral subtraction technique. In this prior-art method, the instantaneous or short-time spectrum of the disturbed signal is determined according to its magnitude (or power) and phase by means of a FFT spectral transformation (fast Fourier transform). The magnitude or value (or power) spectrum is corrected substantially by subtracting an estimated amount (power) spectrum of the interference. Subsequently the phase of the disturbed signal is added, and an inverse FFT is effected. This known method is disclosed for example in an article in IEEE-Transactions, Volume ASSP-26, 1978, pp. 471-472 or Volume ASSP-27, 1979, pages 113-120.
The problem of echo or feedback occurs for example in telephone transmission. Due to reflections on branching circuits, interference echoes are produced which are to be cancelled by adaptive filters. An adaptive filter can also be employed for suppressing feedback in a so-called loud telephone.
For the purpose of signal masking or extraction, the signal spectrum can be split into several frequency bands, and the latter can be mutually exchanged or brought into a reverse position according to their frequencies. In this manner a transmission channel can be ensured against unauthorized wiretapping (German patent publication DE-AS 1 273 002).